The present invention relates generally to pattern generation systems. More specifically, the present invention relates to a column in a lithographic pattern generation system that employs a raster scanned beam writing technique.
FIG. 1 shows an exemplary prior art column 10 employed in a photolithographic pattern generation system that includes a high brightness electron source 12 such as a ZrO Schottky emission cathode with extraction energy of 10 kV. Source 12 produces an electron beam 14 that is directed along a path 16. Disposed in path 16 are a focusing lens 18, a first stop 20, a second stop 22 and an objective lens 24. First stop 20 includes a square aperture 20a that lies in path 16, and second stop 22 includes a rectangular aperture 22a that lies in path 16. Disposed between first stop 20 and second stop 22 is a first deflector 26. A second deflector 28 is disposed between second stop 22 and objective lens 24.
Lens 18 may be a series of magnetic lenses or electrostatic lenses and is used to focus electrons in beam 14 to pass through square aperture 20a. First deflector 26 deflects beam 14 through an angle xcex8d with respect to second aperture 22a, systematically allowing a portion of beam 14 to propagate through objective lens 24, discussed more fully below.
Objective lens 24 defines an object plane 30 located between first deflector 26 and second deflector 28, proximate to second stop 22. Although object plane 30 is shown positioned between first deflector 26 and second stop 22, object plane 30 may be positioned between second deflector 28 and second stop 22. Objective lens 24 images object plane 30 onto an image plane 32. Beam 14 impinges upon image plane 32 as a shadow, as opposed to a focused image, of the overlay of square aperture 20a and rectangular aperture 22a. With this configuration, the area of the shadow impinging upon image plane 32 is determined by the focus of objective lens 24 instead of image magnification. The area of the shadow may be much smaller than the physical size of either first aperture 20a or second aperture 22a, and its size may be adjusted by varying the distance between the cathode crossover 16a and object plane 30. Positioned in object plane 30 is a substrate 36, upon which a pattern is written.
When writing a pattern, it is desireable to provide the highest quality pattern in a minimum amount of time, which is expressed in terms of the pattern coverage rate (R). R specifies the pattern area exposed per second of writing time. R is normally expressed having the dimensions of square centimeters per second (cm2/sec). Thus, it is desireable to employ a pattern writing technique having a high R.
One such pattern writing technique is described by Rishton et al. in Raster shaped beam pattern generation J. Vac. Sci. Tech. B17:6, p. 2927 (1999) and employs a graybeam data format to specify a fraction of patterned area within each pixel on a regular grid. The pixel grid is further partitioned into a flash grid, where flash sites include four graybeam data pixels in a 2xc3x972 array. The beam is scanned periodically over the substrate. An exposure amplitude retrograde scan is added to the uniform saw tooth raster scan, so that the beam appears to dwell on each area of exposure for approximately 10 nsec. At each exposure, the pattern is composed using a shaped beam, allowing edges to be positioned on an address grid that is much finer than the pixel grid. The size and shape of the beam is derived from a 4xc3x974 pixel array of graybeam data surrounding the exposure area. The exposure time is varied between about 30%-80% of the exposure cycle time to correct for proximity scattering and other dose error effects.
Pattern exposure is controlled as a function of the flashing and blanking of beam 14. The flash is a portion of the pattern written in image plane 32 during one cycle of an exposure sequence by the presence of beam 14 in image plane 32. The blank is the absence of beam 14 in image plane 32.
Referring to both FIGS. 1 and 2, a flash occurs when first deflector 26 deflects beam 14 so that a shadow of square aperture 20a superimposes a portion of rectangular aperture 22a, referred to as a flash position 34. A blank occurs when second deflector 28 deflects beam 14 so that no portion of the shadow of square aperture 20a superimposes rectangular aperture 22a, referred to as a blank position 36. First deflector 26 systematically flashes and blanks beam 14 in accordance with the pattern to be written.
Referring to FIGS. 1 and 3, a prior art blanking technique is shown. At the commencement of the flash cycle, the shadow of square aperture 20a impinges upon the surface of stop 22, referred to as blank position 40. During the flash cycle, beam 14 is deflected so that the shadow of square aperture 20a moves along a first direction to a flash position 42, in which a portion 44 thereof superimposes rectangular aperture 22a. At the end of the flash cycle, the shadow of the square aperture returns to blank position 40. To that end, beam 14 is deflected so that the shadow of square aperture 20a moves along a second direction, opposite to the first direction.
A drawback with this blanking technique is that it results in an undesirable xe2x80x9cshutterxe2x80x9d effect, due to the limited bandwidth and settling time of the deflection drive electronics. Specifically, beam 14 impinges upon regions of substrate 36 that should not be exposed when proceeding to the final position. In addition, the presence of a single blanking position, such as blank position 40, results in regions of substrate 36 being exposed longer to beam 14 than other regions. The net result is a non-uniform dose distribution that causes errors in both the location and size of pattern features.
Referring to FIGS. 1 and 4, shown is another prior art blanking technique for an alternate embodiment of stop 122. Stop 122 includes four apertures 122a, 122b, 122c and 122d. Beam 14 is deflected so that the shadow of square aperture 20a moves back and forth in opposite directions when traveling between a blank position 140 and a flash position 142. The choice of aperture 122a, 122b, 122c and 122d selected for a flash position depends upon the shape of the region on the substrate to be exposed. This depends upon the relationship between pattern features to be written and the flash grid. As discussed above with respect to FIG. 2, this blanking technique also results in non-uniform dose distribution.
What is needed, therefore, is a blanking technique that provides improved dose uniformity.
Provided is technique for generating patterns with a photolithographic system that employs a multiple blank position flash cycle. In accordance with one embodiment of the present invention, a beam, creates a shadow of a first aperture that impinges upon a region of a stop, referred to as a first blank position. The beam is deflected so that the shadow of the first aperture moves along a first direction A to a flash position, in which a portion thereof superimposes a second aperture that is located in the stop. To complete the flash cycle, the beam is deflected so that the shadow of the aperture impinges upon a second region of the stop, referred to as a second blank position. As a result, during the flash cycle, the beam is deflected in one direction to impinge upon two different blank positions. During a subsequent flash cycle, the beam moves the shadow of the first aperture along a second direction, which is opposite to first direction. In this manner, the shadow of the aperture moves from blank position and impinges upon the aperture of the second stop. Thereafter, the beam is deflected to move the shadow of the first aperture of the first stop, along the second direction, from impinging upon the second aperture located in the stop to impinge upon the first blank position.